UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 29, 1994
NRC INFORMATION NOTICE 91-45, SUPPLEMENT 1: POSSIBLE MALFUNCTION OF
WESTINGHOUSE ARD, BFD, AND NBFD
RELAYS, AND A200 DC AND DPC 250
MAGNETIC CONTACTORS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice supplement to alert addressees and vendors of equipment used by the
addressees about continuing malfunctions of Westinghouse NBFD relays. These
relays are used extensively in safety-related applications at nuclear power
plants with nuclear steam supply systems supplied by Westinghouse as well as
other vendors. It is expected that recipients will review the information for
applicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written
response is required.
Description of Circumstances
On July 19, 1993, Northern States Power Company, the licensee for Prairie
Island Nuclear Generating Plant conducted a monthly surveillance test on the
Unit 2 reactor protection system. During the test, two normally energized
Westinghouse NBFD relays would not re-energize. The coils of the relays were
found to have opened electrically. The licensee returned the failed relays to
Westinghouse for failure analysis.
On September 24, 1993, during periodic surveillance testing of the Unit 2
reactor protection system, a third NBFD relay failed (open coil). During an
inspection of the relay racks prior to testing, Northern States Power Company
identified three relays containing longitudinal cracks in their coil cases.
There was also evidence that potting compound that encapsulates the relay coil
had extruded from these cracks; however, these relays operated satisfactorily
during the surveillance test. The licensee manually verified that the plunger
in each of the relays moved freely. Northern States Power Company sent the
failed relay and one of the three relays with cracked cases to Westinghouse
for failure analysis.
9407220187. IN 91-45, Supp. 1
July 29, 1994
Page 2 of 4
On September 27, October 14, and October 21, 1993, Northern States Power
Company conducted additional visual inspections of the reactor protection
system relay racks, identifying 21 more relays that had cracked coil cases.
One of the cracked cases was on a normally de-energized relay. Following the
shutdown of Prairie Island Unit 2 on October 28, 1993, for a refueling outage,
the licensee removed all Unit 2 NBFD relays from service. An additional 36
relays were discovered to have cracked coil cases. Northern States Power
Company speculates that many of the cracks existed previously, but could not
be seen because of the difficulty in inspecting installed relays. In total,
Northern States Power Company identified 27 relays with cracks of varying
lengths in train A and 33 relays in train B. There are 43 relays in each
train. Northern States Power Company speculated that the cracked coil cases
were indicative of a potential premature failure of the relay and therefore
all relays with cracked coil cases were replaced.
Northern States Power Company discussed the issue of the cracked relay cases
with the Wisconsin Public Service Corporation, the licensee for the Kewaunee
Nuclear Power Plant. Wisconsin Public Service Corporation visually inspected
its installed reactor protection system relays and identified 11 cracked
relays among the 80 relays in the Kewaunee Nuclear Power Plant system.
Wisconsin Public Service Corporation predicted that other relays with cracked
cases likely existed, but could not be identified because of the difficulty
associated with visual inspection of installed relays. The Kewaunee licensee
replaced all relays that were found to have cracked cases.
Discussion
NRC Information Notice 91-45 transmitted to all power reactor licensees a
Westinghouse 10 CFR Part 21 report to the NRC (Westinghouse Report No. NS-NRC-
91-3600 dated June 24, 1991). The Westinghouse report described the potential
for malfunction of certain products that are supplied by Westinghouse
(including NBFD relays). The potential for malfunction arose from the failure
of the epoxy potting compound used to encapsulate the coils of the affected
products. The potting compound on certain devices was thought to have been
incorrectly manufactured. The failure mode involved the flow of semi-fluid
potting compound into the armature of the affected device. This would cause
the armature plunger to move sluggishly, resulting in delayed reset of the
device.
The Westinghouse report stated that the issue was limited to continuously
energized, direct current devices. The heat from continuously energized
devices was thought to be sufficient to soften non-homogeneous potting
compound. According to Westinghouse, a continuously energized relay would
exhibit deficient potting compound within two surveillance cycles. For
devices that had not been in-service for at least two surveillance cycles,
Westinghouse recommended either replacement of the suspect devices with
devices that had been subjected to updated dedication procedures and processes
to identify any non-homogeneous potting compound, or conducting a specific
inspection of the epoxy potting compound following continuous energization for
a period of two hours (or simulated service conditions). Any device that
exhibited evidence of epoxy softening should be discarded.
IN 91-45, Supp. 1
July 29, 1994
Page 3 of 4
On December 13, 1991, Westinghouse issued a supplement to the original
Westinghouse Part 21 report. Westinghouse recommended that all spare relays
be tested at 250 degrees F (121.1 degrees C) for 2.5 hours in a preheated oven
and then checked for epoxy softening (referred to as the heat-and-probe test).
Westinghouse concluded that if the epoxy did not soften as a result of this
test, then the device was suitable for continued in-service application.
When the Westinghouse 10 CFR Part 21 report was issued, all the relays in the
Prairie Island Unit 2 reactor protection system had been in service for at
least two surveillance cycles, and were therefore not tested or replaced.
Unit 2 relays with the case cracking problems and the relay with the open coil
failure had all satisfied the original Westinghouse Part 21 in-service
acceptance criteria. In Prairie Island Unit 1, 11 new relays were installed
in the reactor protection system subsequent to the Westinghouse Part 21
report. These relays had been satisfactorily heat-and-probe tested as
recommended by the Westinghouse Part 21 supplement report issued December 13,
1991. The remaining Prairie Island Unit 1 RPS relays were purchased and
installed after Westinghouse had corrected the inadequate process that had
resulted in the manufacture of the non-homogeneous potting compound. There
have been no similar open coil failures or observed cracking of Prairie Island
Unit 1 RPS relay coil cases.
The Westinghouse analysis of the two relay coils that failed on July 19, 1993,
at Prairie Island, stated that the root cause of the open coil failures was
deficient potting compound. The coil failures were attributed to the normal
inductive voltage spike that is generated when the coil is de-energized. This
spike, about 2000 volts, was thought to be sufficient to breach degraded coil
insulation, resulting in coil failure. The insulation degradation was
attributed to heat and poorly manufactured potting compound. However, in
their failure analysis report for the September 24, 1993, open coil failure,
Westinghouse stated that improper or inconsistent application of Mylar
insulation during the manufacturing process may have contributed to the
failures of all three relay coils.
According to Westinghouse, the issue involving the improper sizing and
positioning of Mylar insulation was identified in July 1991. Westinghouse
stated that at that time the proper installation of the Mylar insulation was
verbally stressed to manufacturing personnel. In May 1992, proper Mylar
insulation sizing and specific positioning was, for the first time, referenced
on applicable manufacturing drawings. The size and thickness of the Mylar
insulation was increased and the importance of Mylar finish wrap size and
positioning was re-emphasized to manufacturing personnel.
Regarding the coil case cracking phenomenon observed at Prairie Island and
Kewaunee, Westinghouse stated that the coil case cracking phenomenon is likely
caused by thermal expansion. The coil case is molded from a stable phenolic
material but is thin-walled and thus subject to stress cracking when exposed
to temperature extremes and internal pressures. Westinghouse stated that in
laboratory tests, only 4 relays out of 1059 NBFD relay coils that were heat-
and-probe tested for potting compound integrity, exhibited coil case cracking.
Further, Westinghouse stated that few NBFD relays returned for failure
analysis have had cracked coil cases. The Westinghouse experience does not
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July 29, 1994
Page 4 of 4
appear consistent with the experience of the Prairie Island and Kewaunee
licensees. Collectively, Northern States Power Company and Wisconsin Public
Service Company have identified 74 of 166 NBFD relays with cracked coil cases.
The Prairie Island and Kewaunee licensees have not identified any unusual
operating or environmental conditions that would account for the relay case
cracking or coil failures. Although the cracked coil cases may not be
indicative of a premature relay failure, the NRC staff is concerned that the
number of cracked relay cases found by the Prairie Island and Kewaunee
licensees is greater than previous Westinghouse experience indicated.
NBFD relays may fail due to improper or inconsistent application of Mylar
insulation placement alone or in conjunction with potting compound flow. In
addition, potting compound flow may cause increased internal pressure which
may result in coil case cracking. Westinghouse has stated that the
heat-and-probe test and two surveillance cycle acceptance criteria were
intended only to detect deficient potting compound. These tests may,
therefore, be inadequate to identify NBFD relays which are subject to
premature failure.
This information notice supplement requires no specific action or written
response. If you have any questions about this matter, please contact one of
the technical contacts listed below or the appropriate Office of Nuclear
Reactor Regulation project manager.
original signed by A. E. Chaffee
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: M. Dapas, RIII
(612) 388-8209
N. Fields, NRR
(301) 504-1173